Catalyst components for the stereospecific polymerization of olefins, such as propylene, are widely known in the art and they are of the Ziegler-Natta category type. The first catalyst of this type widely used in the industry was based on the use of solid TiCl3 obtained by reduction of TiCl4 with aluminum alkyls. The activity and stereospecificity of the catalysts were not so high so that the polymer had to be subject to a deashing treatment to remove the catalyst residues and to a washing step for removing the atactic polymer produced. Nowadays, the most spread out catalyst family used industrially comprises a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound. Conventionally these catalysts are used together with an external donor (for example an alkoxysilane) which helps in obtaining higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of phthalic acid, diisobutylphthalate being the most used. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system is capable of giving good performances in terms of activity, and propylene polymers with high isotacticity and xylene insolubility. It is however of general interest the possibility of increasing the intrinsic capability of the solid catalyst components, particularly of those based on donors different from phthalates, to produce stereoregular polymers. In fact, an intrinsically more stereospecific catalyst component would allow to use a lower amount of stereoregulating external donor to reach the target of polymer xylene insolubility and this, in turn, would be translated into the possibility of obtaining a higher plant productivity.
Based on this, it would be very convenient to find a way of improving the stereospecificity of a solid catalyst component and in particular it would be convenient that this method be of a wide applicability.
Since the discovery of magnesium chloride based supports numerous attempts have been made to include in it additional compounds with the aim of imparting new or modified properties to the final catalysts.
In U.S. Pat. No. 4,613,655 substantial amounts (30% by weight or higher) of different inorganic compounds and, among them Cu2Cl2 (table VII), is mixed with MgCl2 and then ground in the presence of TiCl4 in order to produce a catalyst. Apart from the effect of dilution of MgCl2, the catalyst, used in the ethylene polymerization, the catalyst did not show any improvement from the presence of Cu2Cl2.
JP2010-155949 discloses the preparation of solid catalysts components according to several techniques all of them having in common the use of copper containing compounds at various stages of preparation. Depending on the preparation technique and specific ingredients, the final amount of Cu in the catalyst (table 1 and 2) and its relative ratio with Ti may vary from Cu/Ti weight ratio of 1.91 (Ex. 1, the highest) to 0.55 (Ex. 7, the lowest).
According to this reference, the catalyst components containing Cu allows to get increase in the catalyst activity while the stereospecificity is maintained at the same level of the comparative catalyst (not containing Cu) or slightly increased. This is confirmed by the review carried out on examples 1-3 and comparative example 1 in table 1 (all catalyst prepared with the same technique) showing that an increase in stereospecificity with respect to the catalyst not containing Cu (Comparative 1; CXS 2.3) was obtained only for the catalysts (1-2) in which the weight ratio Cu/Ti was higher than 1 and in particular the trend clearly shows a linear decrease of stereospecificity (expressed by increasing values of CXS i.e., amount of low crystallinity soluble matter) going from Cu/Ti weight ratio of 1.91 (EX. 1 CXS1.1) to weight ratio Cu/Ti 0.77 (EX3, CXS 2.3 as in the comparative example 1). It has to be reported that when the catalyst is prepared according to a different technique (Example 4) the increase in sterospecificity with respect to catalyst not containing Cu (comparative example 2) is not seen at all. In addition, when the catalyst is prepared according to a still different technique and the Cu/Ti weight ratio is 0.55 the stereospecificity with respect to the same catalyst without Cu (comparative example 5) is even slightly worsened.
The same teaching described above is also reported in JP2010-155948 and JP2010-155950.
Now the applicant has surprisingly found that it is possible to increase the stereospecificity of catalyst components based on Mg containing support on which are supported titanium atoms and donors by modifying it with specific amounts of Cu compounds.